The present invention relates to a means for cleaning liquid suspensions, especially fiber suspensions used in paper manufacturing. Apparatuses of the type according to the present invention are generally constructed and used as a part of hydrocyclones, which due to their generally simple construction and lack of movable parts have proved to be especially suitable for this kind of cleaning.
Cleaning of fiber suspensions by means of hydrocyclones, on a so called centrifugal principle is in many cases a complicated process, since the majority of the particles being separated have a density (specific weight) very close to each other. On one hand, there are the "useful" particles, such as cellulose fibers, and on the other hand there are various fouling particles, plastic particles and other impurities. Both groups have densities so close to each other that the separation thereof in one stage becomes difficult. Therefore, cyclones are mounted in a combined system (for example, using so called cascade coupling), wherein fiber suspensions are recirculated through the cyclones and thus subject to multi-stage cleaning.
A hydrocyclone operates, as known, by receiving the liquid to be cleaned, the so called feed, and by rapidly rotating it so that the lighter particles accumulate at the center, whereas the heavier particles approach the periphery, due to the centrifugal forces acting on the particles of differing density. The circulating liquid is distributed and discharged from the cyclone such that the major portion escapes through a central discharge opening at the near end of the cyclone with respect to the feed, forming a so called accept, whereas the portion with the separated particles circulating in the periphery is transported towards the bottom of the cyclone to be discharged as a so called reject. As described above, separation can seldom be carried out completely in a single stage or cyclone passage, since the reject still includes separable useful particles which should not be wasted and thus the cleaning continues from apparatus to apparatus in the described manner.
Cyclone type cleaning methods are concerned with the separation of particles, the density of which are close to each other as discussed above. In mixed cascade connected systems, the coarser and heavier particles that occur in the feed must also be discharged, for example sand, metal particles and other heavier impurities. These are together termed the coarse particles. The probability that these will cause problems may be quickly eliminated by slinging them towards the periphery of the cyclone in order to be discharged with the reject. These heavy particles are entrained in the subsequent cleaning stages and are concentrated continuously, remaining in the final reject, which includes all matter that is separated from the completely treated fiber suspension.
This final reject is not immediately discharged because it still includes useful particles, such as coarser fibers and fiber bundles, so called shives, which would be worth recovering. However, prior to recovery, the amounts of coarse particles that occur in the final reject, i.e. sand, metal particles and other coarser scrap must first be discharged, because they have a harmful influence on the means which will treat the reject material, for example on pumps, but above all, on grinding means and refiners. It is also important that the coarse separation is carried out without a large pressure loss. In other words, it should not cost much to remove the coarse particles in order to collect said rest of the usable fibrous material.
There is as such, of course, no problem in separating coarse particles from a liquid or fiber suspension in a cyclone; as already mentioned, the heavier particles are rapidly slung out towards the cyclone wall to glide downwards there along and to be discharged with the reject. It is undesirable to lose some of the liquid with the reject, but rather the discharge is only to get rid of the coarse particles, while the rest of the liquid continues its flow through the system. Known apparatuses use devices often called as "sand traps", see, for example, U.S. Pat. Nos. 3,259,246 or 3,529,724. The reject with the coarse particles is therefore allowed to flow out to a closed chamber, where the particles accumulate while the suspension liquid is delivered in one way or another back into the cyclone through the central part of the reject outlet. A low pressure condition prevails there, drawing the suspension into the cyclone again, from where it flows up along the center of the cyclone and is discharged either through the accept outlet or the reject outlet.
These known apparatuses have in principle two disadvantages. First, when installed in a piping system with flowing liquid so as to form, for example, "sand traps" they produce considerable pressure loss. Second, their separation efficiency is insufficient. Particles which are definitely "coarse", i.e., having a high density, but on the other hand are so small that they possibly pass through, will remain in the accept. This concerns especially particles which are very hard, for example, sand or quartz particles, which can cause difficulties in further processing.